1. Field of the Invention
The present invention generally relates to a digital watermarking technique. More particularly, the present invention relates to a digital watermarking technique for embedding or reading digital watermark data in digital data contents which represent an image or a sound. In addition, the present invention relates to a technique for statistical processing of read watermark data in a system using the digital watermarking technique.
It is easy to replicate or tamper fraudulently with multimedia production, and the easiness hinders an data content provider from sending data. In addition, some users may not use the data originated from the provider validly. Therefore, copyright protection is strongly needed for the multimedia production. The digital watermarking technique is effective in realizing the copyright protection. According to the digital watermarking technique, sub-data is embedded in data contents without being noticed by a user by utilizing redundancy of data such as of an image and a sound. The digital watermarking technique is used for protecting a multimedia copyright by embedding copyright information, a user ID and the like as the sub-data in secret, since it is difficult to separate the sub-data from the data contents.
2. Description of the Related Art
Conventionally, the following digital watermarking techniques are proposed.
According to a technique proposed in Japanese patent application No. 9-57516, “Image processing method and the apparatus,” an image is subdivided into blocks larger than a 8×8 block size which is used for common non reversible compression. Then, the size of the frequency coefficient which is obtained by discrete Fourier transform of the block is changed, the frequency coefficient being represented by a polar coordinate system and the size being a distance from the origin point of the polar coordinate system. As a result, sub-data can be read correctly even when the non-reversible compression is performed. In addition, the frequency coefficient is normalized within a range of predetermined values, is embedded, and read. In addition, weaker image processing is carried out on a complicated region as compared to a flat region. As a result, degradation of image quality which may be caused by embedding the sub-data can be suppressed and a tolerance to contrast changing is obtained. Further, as the value of the frequency coefficient to be changed becomes larger, the modification amount of the frequency coefficient becomes larger (the smaller the value is, the smaller the modification amount is) so as to suppress the deterioration of image quality more effectively. In addition, when subdividing an image into blocks, an image area which is smaller than one block is treated as one block by using an average pixel value and/or using a form symmetric with respect to a line repeatedly to compensate for the lacking image area. Moreover, the sub-data is constituted from the whole image after weighting data of each block. As a result, the sub-data is read correctly even when the image is partly edited and/or the image with many flat parts is non-reversibly compressed.
In addition, according to a technique proposed in Japanese patent application No. 9-164466, “Information embedding method, data reading method and the apparatus,” when embedding data into motion pictures, data embedding is carried out to components of a relatively low frequency region. Further, frequency conversion is carried out with a block size larger than a block size used for data compression, and, then data embedding is carried out. Moreover, an original image is used when data is read. As a result, tolerance to data compression is obtained.
Other conventional techniques are proposed in Japanese patent applications No. 8-305370, No. 8-338769, No. 9-9812, No. 9-14388, No. 9-109924, No. 9-197003, No. 9-218467 and No. 10-33239. The digital watermark method is also called data hiding, finger printing steganography, image/sound deep encryption and the like.
Elements for determining performance of the digital watermarking technique are as follows:                (1) quality of data contents in which the digital watermark is embedded;        (2) durability of the digital watermark which is embedded in the data contents when the data contents are manipulated;        (3) safety against intentional erasing of and tampering with digital watermark data, and        (4) reliability of the digital watermark data which is read from the data contents.        
The digital watermarking technique is broadly divided into two methods. One method of gives meaning to a data value by quantizing. For example, by dividing a data value by a quantization value and dividing the result by 2, a bit data can be represented by the remainder. Another method embeds digital watermark data by using a spread spectrum method.
The above-mentioned examples are based on the former method. In terms of the method, there is a problem with respect to the above element (1) in that the digital watermark data embedded in the data contents may be perceived, or commercial value of the data contents may be lost by embedding the digital watermark data. With respect to the above element (2), the digital watermark data which is embedded in the data contents may be dissipated even when a general user uses the data contents in a normal way. Particularly, it is a difficult problem to achieve both elements (1) and (2) with enough performance in practical use.
In addition, there is a method of embedding the digital watermark data repeatedly in order to give durability to the digital watermark data against manipulation of the data contents. Specifically, according to the method, digital watermark data which is embedded repeatedly (which is called a watermark sequence hereinafter) is read from data contents, and, then, the digital watermark data is reconstituted by performing statistical processing. The watermark sequence has durability against deterioration and noise to some extent. However, if the data contents are encoded by high compression rates, it may become difficult to read the watermark sequence from the data contents. Therefore, it may become impossible to reconstitute the digital watermark data.
In addition, as for a digital watermarking system, accuracy for determining the presence or absence of embedded data is important. In addition, reliability of embedded data is important. The digital watermarking system generally has a mechanism for reconstituting correct digital watermark data even when sub-data embedded in the data contents is corrupted to a certain extent, since the digital watermarking system assumes various processing on the watermarked data contents. However, under present circumstances, it is impossible for the system to evaluate validity of reconstituted digital watermark data quantitatively. Therefore, the system does not have enough reliability.